1. Field of the Invention
This invention relates to a spark plug for a gas engine which is driven by gaseous fuel such as natural gas, city gas, liquefied petroleum gas or the like.
2. Description of Prior Art
In a spark plug for a gas engine which is driven by gaseous fuel such as natural gas, city gas, liquefied petroleum gas or the like, it has been used as a driving means for an air-conditioner, an electrical generator and the like. In this type of gas engine, it has been demanded to prolong its endurance (service life) for a long consecutive operation and an extended intervals between maintenance working.
It, however, requires a high voltage for a spark discharge to occur across a center and a ground electrode due to its high compression pressure and gaseous fuel used for the gas engine. This causes the temperature of the center electrode to excessively rise so as to spark erode a firing end of the center electrode due to its intensified oxidation. In order to prolong the service life of the spark plug for the gas engine, it has been necessary to narrow a spark gap between the electrodes so as to drop the required discharge voltage therebetween while providing a noble metal tip on the firing end of the center electrode.
Simply narrowing the spark gap worsens an ignitibility due to a flame extinguishing effect. Merely thinning the noble metal tip diametrically in order to improve the ignitibility causes its temperature to extraordinarily rises due to concentration of spark discharge energy, and thus spark eroding the noble metal tip because of the intensified oxidation.
On the other hand, when a platinum tip is employed with the spark gap narrowed, the metal grains deposit (sweating) successively on a discharge surface of the tip with the repetition of the spark discharge owing to spark discharge energy and combustion gas of high temperature. Further repetition of the spark discharge grows the grains to bridge the narrowed spark gap.
Therefore, it is an object of the invention to provide a spark plug for a gas engine which is capable of prolonging a service life without losing a good ignitibility against gaseous fuel, and at the same time, controlling the spark discharge energy to avoid the spark erosion of the noble metal tip due to an oxidation-based evaporation.
It is another object of the invention to provide a spark plug for a gas engine which is capable of preventing the bridging of the spark gap so as to avoid the occurrences of inadvertent misfire when the spark gap is adapted to be narrowed.